Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended
	Dec. 29, 2024	Dec. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 778	$ 74,980
Other comprehensive (loss) income, net of tax
Foreign currency translation adjustment, net of tax	(108,846)	63,106
Net pension adjustments	(33)	(13)
Other comprehensive (loss) income, net of tax	(108,879)	63,093
Comprehensive (loss) income, net of tax	(108,101)	138,073
Less: Comprehensive income attributable to noncontrolling interests, net of tax	31	8
Comprehensive (loss) income attributable to Tetra Tech, net of tax	$ (108,132)	$ 138,065